Start by putting the brown sugar and corn syrup on the stove to boil. While you're waiting, mix the oats and cereal together in a bowl and line a 9x13 pan with wax paper. When your mixture boils, remove from heat and add peanut butter and vanilla, stirring until combined. Add to the cereal and oats and stir until the oats and cereal are covered by the peanut butter mixture. If you are adding any extras, wait a few minutes until it's not so hot to stir them in. Dump the mixture into the pan and spread out evenly with your fingers. Wait 15-20 minutes, and then slice with a pizza cutter.